1. Field of the Invention
The present invention relates to a method for preventing decarburization of steel materials during heating.
Steel articles are generally produced by a process comprising heating steel materials in such forms as slabs, beam blanks, blooms and billets in a heating furnace and rolling them. However, scales are formed on the steel materials during their heating, resulting in a lowered product yield, accompanied with various problems, such as lowering in the commercial value of the products due to surface imperfection and lowering in the strength of the steel products due to decarburization.
Particularly in the case of steel materials, such as billets for wire rods or rail rods, which contain about 0.5 to 1.2% carbon, the influence on the steel quality by the lowering in strength due to the surfacial decarburization is very significant.
2. Description of Prior Art
For preventing the decarburization of steel materials during their heating, it has been proposed to apply an oxidation inhibitor on the steel surface, or apply a coating containing carbonaceous material on the steel surface and to keep in a reducing atmosphere for a time long enough for making the coating satisfactorily dense, as disclosed in Japanese Patent Publication No. Sho 42-12335, or to apply a duplex layer coating composed of a first layer of a substance which generates CO or C0.sub.2 and a second layer of an oxidation inhibitor, as disclosed in Japanese Laid-Open Patent Specification No. Sho 49-97736.
However, none of the prior art has been proved to be successful; some requiring a considerably long time for keeping the steel materials in the heating furnace, others failing to produce desired results when the heating temperature is high or when the steel materials which contain a high carbon content.
Thus, the glass-like coating used in the Japanese Patent Publication No. Sho 42-12335, has no ability to completely shield the exterior gas, and it is impossible to avoid the oxidation of carbon in the surfacial layer of the steel materials by oxygen which diffuses through the coating if the heating time is long or high, or if the steel materials are high-carbon materials.
Meanwhile, in the Japanese Laid-Open Patent Specification No. Sho 49-97736, some of the CO or CO.sub.2 generating substances show an exessively rapid decomposition rate, thus losing its decarburization preventing ability at a premature stage, while others are too stable to be decomposed at a desired stage, thus failing to produce a desired decarburization preventing ability. This prior art does not teach a substance having a decomposition rate optimum for the decarburization prevention. Moreover, the method disclosed by this prior publication is completely unable to prevent the decarburization of high-carbon steel materials such as a 1% carbon steel.